This invention relates to digitizers and, more particularly, to digitizers for converting shaft angle to binary numbers.
In recent years an impressive variety of shaft position digitizers have been proposed to satisfy the requirements of high resolution electromechanical systems. There have been great efforts directed to devising methods of increasing the angular resolution of digitizers while, simultaneously, reducing their overall size and weight. The term "resolution" is used here to denote the number of unique binary words provided by the digitizer for 360 degrees of input shaft rotation.
Historically, many digitizers have been devised which employ a single, coded disc or cylinder for converting shaft angle displacement to a binary digital representation of the angular displacement with the requisite resolution. More particularly, in such digitizers the coded element is generally mounted to a rotatable primary shaft, the angular position of which is to be digitized. An exemplary binary code disc has a series of concentric rings, each ring being divided into a number of equal sized units of alternate binary significance. Starting with the innermost ring, the units of which represent the most significant digit of the binary number to be generated, the units of the rings at the greater radial distances from the center of the disc are one-half of the size of the units of the adjacent interior rings. Therefore, the units of each ring at increasingly greater radial distances from the disc center represent a decreasingly less significant digit. The binary number representing the input shaft position is then read from the code disc by a number of sensors, one sensor being individual to each ring. Angular positions of the input shaft are thus converted by the digitizer to unique numbers, one for each definable angular position on the code disc. Obviously, the maximum resolution of this exemplary single disc digitizer is determined by the size of the smallest unit of the outside ring and the ability to accurately and reliably read the smallest unit. To achieve resolutions of a few seconds-of-arc or better with a digitizer employing a single code disc on which the finest units which may be read by conventional methods are positioned at the greatest distance from the center of the disc, the code disc would be too large to be practical for use in most applications. To eliminate such prohibitive size, attempts to achieve better resolution have been centered around the use of least-significant-bit-reading vernier mechanisms and electronic interpolation of least significant bits. Both of these techniques add significantly to the complexity and cost of the digitizer.
Another type of a single coded element digitizer is the subject of U.S. Pat. Nos. 3,168,643 and 3,165,730, issued to Robinson. The Robinson patents disclose single, motor-driven coded discs or cylinders for digitizing preselected functions which vary with a shaft position. While the use of a motor driven coded element provides absolute or whole-word information from a single code ring, it does not achieve the higher resolution so often needed in todays industry.
Yet another type of a single coded element digitizer is the subject of U.S. Pat. No. 3,831,169, issued to Raser. The Raser patent disclosed a single, motor-driven vane for digitizing shaft angles. While theoretically, this patent offers an improvement in resolution, in practice it is limited by the necessity of a constant speed drive motor and a fixed frequency square-wave oscillator, for if the motor speed varied or if the oscillator frequency drifted, gross digitizing errors could result. Another method used to increase resolution is the use of multi-stage digitizers with mechanical speed increasers connected to the input shaft. These digitizers usually consist of a two stage disc encoding mechanism which has a second coded disc that is gear driven at a slower rate than the first so that the binary output word is achieved in many revolutions of the input shaft. More particularly, in such digitizers the first code disc might produce 256 binary counts for 360 degrees of input shaft rotation and the second code disc might produce 32 counts for 360 degrees of rotation. By coupling these discs thru a 32 to 1 speed reducer, the digitizer would provide 8192 binary counts for 32 turns of the input shaft. In digitizer terminology, this exemplary device would be said to have a resolution of 8 bits and a capacity of 13 bits in 32 turns of the input shaft. The resolution of this device could be increased to 13 bits by connecting a mechanical speed increaser with a ratio of 1 to 32 to the input shaft of the digitizer. While the resolution of multi-stage digitizers is, theoretically, increased, concomitant increases in size and weight of the devices, together with serious gear induced inaccuracies, limit the actual resolution attainable. Most particularly, tooth spacing errors on the gears, gear backlash and a relatively large inertia reflected to the input shaft are problems which generally preclude the use of gear-coupled devices. The present state-of-the-art on multi-stage digitizers included U.S. Pat. Nos. 3,419,727 issued to Pabst; 3,808,431 issued to Hedrick, and 3,525,094 issued to Leonard. All of these referenced multi-stage digitizers could be coupled to the primary input shaft thru a speed increaser to increase resolution but the practical resolution achievable would again be limited by the gearing errors previously described.
The present invention contemplates an analog-to-digital shaft angle digitizer which eliminates these limited resolution capabilities of the prior art devices.